1. Field of the Invention
The present invention relates to an image display device such as monitor or television set using a cathode ray tube, and more particularly to a circuit and process for preventing spots from occurring on the center portion of the screen of the cathode ray tube of a variable visual display when switching off electric power.
2. Description of the Prior Art
In general, a cathode ray tube is constructed with a pair of electrically conductive layers made of refined graphite on the inner and outer surfaces of a glass visual display panel. The glass serves as the dielectric of a capacitor having a capacitance of about 800.about.2000 pF and a very high withstand voltage.
When the cathode ray tube is activated, a high voltage is applied to conductive layer on the inner surface of the glass panel. At this state, even though electric power is turned off, remaining heat in the heater of the cathode ray tube keeps on emitting thermoelectrons. When the electric power is turned off, deflection is disable since no electric current to flow in the vertical deflection coil and the horizontal deflection coil. Therefore, an electron beam of the emitted thermoelectrons continues to be accelerated by a high voltage charged in the capacitor formed between conductive layers. The electron beam strikes the center portion of the fluorescent layer of the cathode ray tube and forms a spot on the phosphor coatings. The high voltage charge in the capacitor discharges over a long period; this causes deterioration ofthe fluorescent layer due to the continuous strong stimulus to the fluorescent layer.
Earlier efforts to prevent a spot from occurring on the cathode ray tube when an image display device is switched off apply a high level voltage to the grid of cathode ray tube during the application of the vertical blanking signal. Consequently, an electron beam generated from cathode is blocked by the grid so that a retrace line does not appear on cathode ray tube and since capacitor is connected between the power source and grid of the cathode ray tube, a negative voltage charge held by the capacitor is rapidly applied to the grid for a certain time period to prevent an electron beam from passing through. I have found that there are drawbacks in this approach because the increased capacitance increases production cost of a product since it is very expensive, and space for such a big capacitor must be provided for on a printed circuit board.
Other known circuits for preventing a spot from occurring on a screen of a cathode ray tube are exemplified by the following patents incorporated herein by reference: U.S. Pat. No. 5,677,730 to Tae-jin Park entitled Spot Elimination Circuit For A Cathode Ray Tube; U.S. Pat. No. 5,084,657 to Mitsunori Ueda entitled Spot Killer Circuit Having Blanking Function; and U.S. Pat. No. 4,340,910 to Silverio A. Valdes entitled CRT Spot Supression Circuit.